Players can walk under the cliffs

There was a few spawnable rock/cliff prefabs introduced this wipe for things like this. If you open the f1 console and type "spawn" followed by 1 of the following you'd be able to block it off without using traditional building pieces.

admin_cliff_low_arc
admin_cliff_medium_arc_arid_small
admin_rock_clutter_medium_d
admin_rock_formation_medium_a
admin_rock_formation_small_c
admin_rock_quarry_small_a